This invention relates generally to an induction heating apparatus and, more particularly, to an induction-heating cooking apparatus for performing cooking due to induction heating in which safety and reliability are improved.
FIG. 1 shows an induction heating-cooking apparatus in which a high frequency magnetic field is generated from a heating coil and applied to a cooking pan as a load, Eddy currents are caused in the pan, and heat cooking due to the self-exothermic character of the pan on the basis of the eddy current loss is performed. An apparatus with such an arrangement as shown in FIG. 1 has been conventionally known. That is, this apparatus is a kind of induction heating apparatus, and the principle of induction heating is applied to a cooking apparatus.
In FIG. 1, reference numeral 15 denotes a main body of a cooking apparatus, and a top plate 16 for mounting a cooking pan is attached on the top surface of the main body 15. A base cabinet 17 is formed on the side surfaces of the main body 15. A power switch 2 and a heating power setting knob 18 are provided in the base cabinet 17. Numeral 19a is a power cord and 19b is a power plug.
FIG. 2 shows a control circuit enclosed in the prior art cooking apparatus main body. In FIG. 2, numeral 1 indicates a commercial AC power source, and a rectifying circuit 3 is connected to the power source 1 through the power switch 2. The rectifying circuit 3 comprises a diode bridge 4, a choke coil 5 and a smoothing capacitor 6. A series resonance circuit consisting of a heating coil 7 and a resonance capacitor 8 is connected to an output terminal of the rectifying circuit 3. The heating coil 7 is arranged in a manner such that it faces and is away from the back surface of the top plate 16, mounted on the top surface of the cooking apparatus main body. The collector-emitter of an npn transistor (power transistor) 9, serving as a switching element, is connected in parallel to the resonance capacitor 8, and a damper diode 10 is also connected in parallel thereto. That is, an inverter circuit for exciting the above resonance circuit is constituted by the rectifying circuit 3, transistor 9, damper diode 8, and a driving circuit mentioned later, etc. The base of the transistor 9 is connected to a driving circuit 11, and the transistor 9 is turned on or off by the driving circuit 11, namely, the inverter circuit is made operative by the driving circuit 11, thereby allowing the resonance circuit to be excited. DC power supplying circuit 20, serving as what is called an auxiliary power source, is connected between the connecting points of the prior art power switch 2 and rectifying circuit 3. The DC power supplying circuit 20 supplies DC voltage for the operation to the driving circuit 11 constituting the inverter circuit, an oscillation controlling circuit 40, an on-off duty ratio determining circuit 50 and a circuit 60 for detecting a fluctuation in the AC power source, which will be explained later, etc. Further, a current transformer 30 is provided between the connecting points of the power switch 2 and rectifying circuit 3. The output of the current transformer 30 is supplied to the oscillation controlling circuit 40. The oscillation controlling circuit 40 has a function to detect the presence or absence and material of the load, i.e., cooking pan, in response to the output of the current transformer 30. When the load is proper, the circuit 40 outputs a power setting signal A at a level corresponding to a set value of a heating power setting volume 41 (which is interlocked with the heating power setting knob 18). At the same time, the circuit 40 outputs a saw wave signal B synchronized with the timing of the oscillation of the foregoing resonance circuit through a transformer 12 for extracting the voltage across the heating coil 7.
On the other hand, the on-off duty ratio determining circuit 50 comprises: a series member of resistors 51 and 52 to which a DC voltage +Vdd is applied; an npn transistor 54 whose base-emitter is connected to the resistor 52 through a resistor 53 and whose collector is connected to the driving circuit 11; a series member of a resistor 55 and a capacitor 56 to which the DC voltage +Vdd is applied; a resistor 57 connected in parallel to the capacitor 56; and a comparator (operational amplifier) 58 in which the voltage developed at a mutual connected point of the resistors 55 and 57 and capacitor 56 is supplied to a non-inverting input terminal (+), and an output terminal is connected to a mutual connecting point of the resistors 51 and 52. The power setting signal A is supplied from the oscillation controlling circuit 40 to a mutual connecting point of the resistors 55 and 57 and capacitor 56. Further, the saw wave signal B is supplied from the oscillation controlling circuit 40 to an inverting input terminal (-) of the comparator 58.
The circuit 60 for detecting a fluctuation in the AC power source comprises: a series member of resistors 61 and 62 to which the voltage across the smoothing capacitor 6 in the rectifying circuit 3 is applied; a series member of resistors 63 and 64 to which the DC voltage +Vdd is applied to an npn transistor 65 whose base-emitter is connected to the resistor 62 and whose collector is connected to a mutual connecting point of the resistors 63 and 64; and an npn transistor 66 whose base-emitter is connected to a mutual connecting point of the resistors 63 and 64 and whose collector is connected to a mutual connecting point of the resistors 55 and 57 and capacitor 56 in the on-off duty ratio determining circuit 50.
Therefore, when a cooking pan 70 is placed on the top plate 16 and the power switch 2 is turned on, the voltage across the smoothing capacitor 6 in the rectifying circuit 3 increases. In the circuit 60 for detecting a fluctuation in the AC power source, when the voltage at the mutual connecting point of the resistors 61 and 62 reaches a predetermined level, the transistor 65 is turned on. When the transistor 65 is turned on, the transistor 66 is turned off. On the other hand, in the on-off duty ratio determining circuit 50, the transistor 66 is turned off, so that the capacitor 56 is charged in response to the power setting signal A from the oscillation controlling circuit 40, causing the noninverting input voltage to the comparator 58 to be increased. In this way, the voltage at the level corresponding to the power setting signal A and the voltage of the saw wave signal B are compared by the comparator 58. The transistor 54 is turned on or off in accordance with the results of the comparison. In this case, when the level of the power setting signal A is higher, the on-off duty ratio of the transistor 54 also becomes high (on-duration becomes long). When the level of the power setting signal A is lower, the on-off duty ratio of the transistor 54 also becomes low (on-duration becomes short). The driving circuit 11 drives and turns on or off the transistor 9 synchronously with the on-off operation of the transistor 54. When the transistor 9 is turned on or off, the resonance circuit oscillates in association with this on-off operation, so that a high frequency current flows through the heating coil 7. In this way, the high frequency magnetic field is generated from the heating coil 7 and is given to the pan 70, so that the eddy current is developed in the pan 70, and the pan 70 generates the heat by itself due to the eddy current loss.
On the other hand, when the power source voltage drops for some reason such as in the case where the user erroneously turns off the power switch 2 during the cooking operation and immediately turns it on afterwards, where defective contact of the power plug 19b occurs, or where instantaneous power failure of the power source itself occurs, etc., the voltage of the smoothing capacitor 6 in the rectifying circuit 3 decreases in response to the voltage drop. When the voltage of the smoothing capacitor 6 becomes less than a predetermined level, the transistor 65 in the circuit 60 for detecting a fluctuation in the AC power source is turned off, causing the transistor 66 to be turned on. When the transistor 66 is turned on, the discharge path of the capacitor 56 in the on-off duty ratio determining circuit 50 is formed, and the non-inverting input voltage level to the comparator 58 becomes zero. As described above, the output level of the comparator 58 becomes low irrespective of the operation of the oscillation controlling circuit 40, and the transistor 54 maintains the off state. The transistor 9 is also turned off in association with turning off the transistor 54, so that the operation of the inverter circuit is stopped, the high frequency current does not flow through the heating coil 7, and the cooking heat is interrupted. When the power source voltage is normally recovered and the voltage of the smoothing capacitor 6 increases, the cooking heat is restarted.
Namely, the oscillation controlling circuit 40 allows the continuation of the operation irrespective of the drop in power voltage, due to the stored voltage capacity of the capacitor in the DC power supplying circuit 20. The circuit 60 for detecting a fluctuation in the AC power source is provided, and the on-off driving of the transistor 9 is immediately stopped by circuit 60 when the power voltage drops. Thus, the operation of the inverter circuit is stopped, thereby preventing the unstable operation of the inverter circuit.
However, since there is a high impedance on the output side of the rectifying circuit 3 when the transistor 9 is turned off, there is a problem that the voltage of the smoothing capacitor 6 does not drop immediately even when the power voltage decreases due to the small load thereacross.
In other words, even when the power voltage drops, if this voltage-drop occur when transistor 9, is off the on-off driving of the transistor 9 will be continued if the output voltage of the DC power supplying circuit 20 does not drop and the operation of the oscillation controlling circuit 40 does not stop. Consequently, the unstable operation of the inverter circuit cannot be always prevented.
Although such continuation of the on-off driving of the transistor 9 is undesirable, when the transistor 9 is turned on due to this continuation of operation, the impedance on the output side of the rectifying circuit 3 becomes low. Therefore, the voltage of the smoothing capacitor 6 certainly drops due to the higher load across capacitor 6. Therefore, depending on the power voltage drop at that time, the on-off driving of the transistor 9 is quickly stopped. Namely, the unstable operation of the inverter circuit can be stopped as the temporary operation.
However, if the user repeats the on-off operation of the power switch 2 many times, or if the power voltage repeatedly fluctuates for a long time due to a defective contact or the like of the power plug 19b, the DC power supplying circuit 20 cannot maintain the operating voltage of the oscillation controlling circuit 40. Thus, operation of the oscillation controlling circuit 40 itself, as the principal part for the control, becomes unstable. When the on-off driving of the transistor 9 continues as mentioned above under such a situation, the operation of the transistor 9 itself becomes unstable. Thus, this causes risks of not only the unstable operation of the inverter circuit but also the breakage of the transistor 9.
On one hand, although the circuit 60 for detecting a fluctuation in the AC power source is inherently a low voltage circuit, it is connected to the inverter circuit as the high voltage circuit. Therefore, parts having high withstanding voltages have to be used as the parts of the detecting circuit 60, so that this also causes a problem of an increase in cost. Further, if parts of the inverter circuit are broken, this breakage will affect the circuit 60 for detecting a fluctuation in the AC power source, causing a risk such that parts of the detecting circuit 60 will have been also broken.